Fluorinated compounds for use in lithium battery electrolytes

ABSTRACT

Provide are fluorinated cyclic and acyclic carbonate solvent compositions such as various fluorine substituted 1,3-dioxolane-2-one compounds and fluorine substituted 1,3-dioxane-2-one compounds, which are useful as electrolyte solvents for lithium ion batteries.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 60/871,076, filed Dec. 20, 2006, the disclosure of whichis incorporated by reference herein in its entirety.

TECHNICAL FIELD

This disclosure relates to electrolyte compositions comprising at leastone partially-fluorinated compound and at least one electrolyte salt. Inother aspects, this disclosure also relates to electrochemical devicescomprising the electrolyte compositions and to articles comprising theelectrochemical devices.

BACKGROUND

The rapid development of electronic devices has increased market demandfor electrochemical devices such as fuel cells and battery systems. Inresponse to the demand for battery systems in particular, practical,rechargeable lithium batteries have been actively researched.Lithium-ion batteries are particularly useful for many portableelectronic devices. Lithium-ion batteries employ highly chemicallyreactive components to provide electrical current. These systems aretypically based on the use of lithium metal, lithiated carbon, or alithium alloy as the negative electrode (anode) and electroactivetransition metal oxides as the positive electrode (cathode).

Lithium-ion batteries are constructed from one or more electrochemicalcells connected in parallel or series. Such cells have consisted of anon-aqueous lithium ion-conducting electrolyte composition interposedbetween electrically-separated and spatially-separated, positive andnegative electrodes. The electrolyte composition is typically a liquidsolution of a lithium salt in a nonaqueous, aprotic organic solvent. Amixture of two or more organic solvents often is used.

The selection of electrolyte solvents for rechargeable lithium batteriesis important for optimum battery performance and safety and involves avariety of different factors. However, long-term chemical stability inthe presence of the charged positive and negative electrodes, ionicconductivity, safety, and wetting capability tend to be importantselection factors in high volume commercial applications.

Long-term chemical stability requires that an electrolyte solvent beintrinsically stable over the battery's range of operating temperaturesand voltages and also that it be either unreactive with electrodematerials or that it contribute to effectively forming a passivatingfilm with good ionic conductivity on the electrodes. Ionic conductivityrequires an electrolyte solvent that effectively dissolves lithiumelectrolyte salts and facilitates lithium ion mobility. From theviewpoint of safety, the characteristics of low volatility, lowflammability, low combustibility, low reactivity toward chargedelectrodes, passivating characteristics, and low toxicity are all highlydesirable. It is also desirable that the battery's electrodes andseparator be quickly and thoroughly wetted by the electrolyte solvent,so as to facilitate rapid battery manufacturing and optimize batteryperformance.

Aprotic liquid organic compounds have been the most commonly usedelectrolyte solvents for lithium batteries. Often, compounds such ascarbonic acid esters (carbonates) have been used, as these compoundstypically share the desirable properties of low reactivity with thepositive electrodes operating at less than about 4.4V vs. Li⁺/Li, lowreactivity with lithium-containing negative electrodes, and athermodynamically favorable solvation interaction with lithium salts,which results in the electrolyte composition having a high ionicconductivity.

The most commonly used aprotic organic electrolyte solvents for use inlithium batteries include cyclic carbonates such as ethylene carbonateand propylene carbonate, cyclic esters of carboxylic acids such asγ-butyrolactone, linear carbonates such as dimethyl caronate, diethylcarbonate and ethyl methyl carbonate, cyclic ethers such as2-methyltetrahydrofuran and 1,3-dioxolane, linear ethers such as1,2-dimethoxyethane, amides, and sulfoxides. A mixed solvent is oftenpreferred in order to balance, or tailor, the desired properties of theelectrolyte composition such as high dielectric constant and lowviscosity.

Drawbacks to the use of conventional lithium battery electrolytesolvents are generally related to their properties such as low boilingpoints and high flammability or combustibility. Some solvents, such asethylene carbonate and propylene carbonate, have boiling points above200° C. However, many electrolyte solvents have boiling points that aresubstantially lower and have flash points less than 30.2° C. (100° F.).Such volatile solvents can ignite during catastrophic failure of a fullyor partially charged battery that has undergone, for example, a rapiddischarge due to a short circuit. Additionally, volatile solventspresent difficulties in the preparation and storage of electrolytecompositions as well as in the addition of the electrolyte compositionto the battery during the manufacturing process. Also, many conventionalbattery electrolyte solvents are reactive towards charged electrodes atelevated temperatures, which can result in thermal runaway under abuseconditions. In fact, recent news reports noted that overheating and evenspontaneous combustion of secondary batteries have led to productrecalls.

SUMMARY

The present inventors noted that non-aqueous electrolyte solutions forlithium ion batteries should have a low viscosity to allow for high ionmobility during high rates of charging and discharging.

The present inventors recognize that there remains a need forelectrolyte solvents with one or more of various advantages, such asreduced volatility, low flammability, and low combustibility relative toconventional solvents, that are less reactive with the charged positiveelectrode and the charged negative electrode, and that effectivelydissolve and dissociate salts to form stable electrolyte compositionsthat adequately wet electrochemical device components and that exhibitadequate ionic conductivities over a range of operating temperatures.Preferably, various embodiments of the present invention have acombination of these advantages. Some embodiments of the presentinvention may have all of these advantages.

Briefly, in one aspect, provided is a solvent composition according toFormula I:

wherein each of the groups R_(h) ¹, R_(h) ² and R_(h) ³ is independentlyhydrogen or C_(x)H_(2x+1), x is an integer from 1 to 4, Y is a singlecovalent bond or the group —CR_(h) ⁴R_(h) ⁵—, each of R_(h) ⁴ and R_(h)⁵ is independently a hydrogen or an alkyl group having 1 to 4 carbonatoms. Also in this formula, R_(f) is —CFR_(f) ¹CHFR_(f) ², whereinR_(f) ¹ is F, or C_(n)F_(2n+1), and n is an integer from 1 to 8, R_(f) ²is F, a linear or branched C_(p)F_(2p+1), wherein p is an integer from 1to 4, or R_(f) ³O(R_(f) ⁴O)_(m), wherein m is 0 or 1, and R_(f) ³ isC_(n)F_(2n+1), and n is an integer from 1 to 8, and R_(f) ⁴ isC_(q)F_(2q), wherein q is an integer from 1 to 4, provided that whenR_(f) ¹ is F and R_(f) ² is F, then at least one of R_(h) ¹, R_(h) ³ andR_(h) ^(a) is C_(x)H_(2x+1), and wherein A is a single covalent bond orCH₂O.

Briefly, in another aspect, provided is a solvent composition accordingto the formula R—O—C(═O)—O—R′, wherein at least one of R and R′ is—C(R_(h) ⁷)(R_(h) ⁸)_(n)CFR_(f) ¹(CF₂CF₂)_(n)CHFR_(f) ⁷, wherein R_(f) ⁷is F, C_(n)F_(2n+1), or R_(f) ³O(R_(f) ⁴O)m-, n is 0 or 1, m is 0 or 1,and R_(h) ⁸ and R_(h) ⁷ are alkyl groups having 1 to 4 carbon atoms, andwherein R_(h) ⁷ and R_(h) ⁸ may together form a ring, provided that whenn is 1 both R_(f) ¹ and R_(f) ⁷ are fluorine.

It has been discovered that at least some of the above-described novelpartially fluorinated carbonate compounds have surprisingly high boilingpoints and low volatilities and thus, in general, are less flammable orless combustible than conventional electrolyte solvents. Yet solventcompositions including the compounds may also quite effectively dissolveelectrolyte salts to provide electrolyte compositions that adequatelywet electrochemical device components (such as separators) and thatexhibit adequate ionic conductivities for use in electrochemical devicesover a range of operating temperatures (for example, from about −20° C.to about 80° C. or even higher, depending upon the power requirementsfor a particular application). The solvent compositions (and electrolytecompositions including the solvent compositions) also can present fewerdifficulties in storage and handling than do some conventionalmaterials, due to one or more relative advantages such as lowervolatility, lower flammability, and/or lower combustibility of theinventive partially fluorinated carbonate compounds and electrolytecompositions.

At least some of the partially fluorinated carbonate compounds areparticularly well-suited for use in large format lithium ion batteries(batteries that operate essentially adiabatically and can thereforeexperience high temperatures, for example, temperatures above 60° C.).In such batteries, electrolyte compositions comprising the compounds ofthe invention can exhibit adequate conductivities, while being lesslikely to react with charged electrodes or ignite during catastrophicbattery failure than some conventional electrolyte compositions.

Thus, at least some solvent compositions including the partiallyfluorinated carbonate compounds meet a need for electrolyte solventsthat have reduced reactivity, volatility, flammability, andcombustibility (relative to conventional solvents), yet effectivelydissolve electrolyte salts to form stable electrolyte compositions thatadequately wet electrochemical device components and that exhibitadequate ionic conductivities over a range of operating temperatures.

In other aspects, this invention also provides electrochemical devices(preferably, batteries) including the electrolyte compositions; andarticles including the electrochemical devices.

In one embodiment, fluorine substituted 1,3-dioxolane-2-one compoundsare provided as electrolyte solvents for lithium ion batteries. Inanother embodiment, fluorine substituted 1,3-dioxane-2-one compounds areprovided as electrolyte solvents for lithium ion batteries. In anotherembodiment, fluorine substituted acyclic carbonates are provided aselectrolyte solvents for lithium ion batteries.

In another embodiment, mixtures of one or more fluorinated compounds ofthe invention together with non-fluorinated cyclic carbonates, acycliccarbonates, more conventional solvents, or combinations thereof, areprovided as electrolytes for lithium ion batteries. In yet anotherembodiment, the invention provides a lithium ion battery with afluorinated carbonate compound or a mixture of fluorinated carbonatecompounds, optionally with non-fluorinated cyclic carbonates and acycliccarbonates and/or a conventional battery solvent as the electrolytesolvent package.

The electrolyte composition of the present invention provides lowreactivity towards charged positive electrodes and charged negativeelectrodes and low flammability. The high boiling points of theelectrolyte compositions of the present invention result in lesspressure buildup in the cells at operating temperatures.

Other features and advantages of the invention will be apparent from thefollowing detailed description of the invention and the claims. Theabove summary of principles of the disclosure is not intended todescribe each illustrated embodiment or every implementation of thepresent disclosure. The figures and the detailed description that followmore particularly exemplify certain preferred embodiments using theprinciples disclosed herein.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

All numbers are herein assumed to be modified by the term “about.” Therecitation of numerical ranges by endpoints includes all numberssubsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3,3.80, 4, and 5).

A non-aqueous electrolyte solvent should have one or more, preferablyseveral desirable properties. Examples follow. A non-aqueous electrolytesolvent should have low flammability or even be nonflammable. Anon-aqueous electrolyte solvent can be nonflammable by having lowvolatility and/or by being very slow to react with oxygen. Onequantitative measure of flammability is flash point. Another measure offlammability involves bringing a lighted match near a weighing pancontaining the solvent of interest and observing whether ignitionoccurs. More details on this measure are found below in the Test Methodsection. Preferably a formulated electrolyte solution has a flash pointabove the operating temperature of a battery in which the electrolyte isused. The operating temperature of such batteries is normally around 60°C. A non-aqueous electrolyte should allow the cell to be charged anddischarged (cycled) without losing capacity after each consecutivecycle.

A non-aqueous electrolyte should be electrochemically stable at highlyreducing and oxidizing potentials. The threshold for electrochemicaloxidation of the non-aqueous electrolyte should be at a voltage (versusLi⁺/Li) that is greater than the cathode (positive electrode) voltage atfull state of charge. The threshold for electrochemical reduction of thenon-aqueous electrolyte should be at a voltage that is lower (versusLi⁺/Li) than the anode (negative electrode) voltage at full state ofcharge.

A non-aqueous electrolyte solvent should be chemically stable to thecharged, active cathode material and to the charged, active anodematerial. One measure of stability is the exotherm onset temperature ofthe reaction between the non-aqueous electrolyte and the charged, activecathode material or the charged, active anode material as determined byaccelerating rate calorimetry (ARC).

However, some limited oxidation or reduction or chemical reaction of thenon-aqueous electrolyte at the electrode surface can be beneficial if apassivating Solid Electrolyte Interface (SEI) layer is formed in theprocess. This SEI layer can protect the electrolyte from furtherreaction with the electrodes.

In one aspect, the present invention provides cyclic carbonatecompositions having structures according to Formula I:

In Formula I, each of the groups R_(h) ¹, R_(h) ² and R_(h) ³ isindependently hydrogen or C_(x)H_(2x+1), wherein x is an integer from 1to 4, and Y is a covalent bond or the group —CR_(h) ⁴R_(h) ⁵ where eachof R_(h) ^(o) and R_(h) ⁵ is independently H or an alkyl group having 1to 4 carbon atoms.

In Formula I, R_(f) can be the partially fluorinated alkyl group—CFR_(f) ¹CHFR_(f) ², wherein R_(f) ¹ is F, or C_(n)F_(2n+1), and n isan integer from 1 to 8. In some embodiments, n is an integer from 1 to 6and in other embodiments, most preferably n is an integer from 1 to 4.R_(f) ² is F, C_(p)F_(2p+1), wherein p is an integer from 1 to 4 andC_(n)F_(2n+1), may be linear or branched, or R_(f) ³O(R_(f) ⁴O)_(m)—,wherein m is 0 or 1, and R_(f) ³ is C_(n)F_(2n+1) wherein n is aninteger from 1 to 8, and wherein R_(f) ⁴ is C_(q)F_(2q) wherein q is aninteger of 1 to 4. In some embodiments, n is an integer from 1 to 6 andin other embodiments, more preferably n is an integer from 1 to 4. R_(f)³ and R_(f) ⁴ may be branched. In some embodiments, R_(f) ² isC_(n)F_(2n+1) or R_(f) ³O(R_(f) ⁴O)_(m)—. In the formula above, whenR_(f) ¹ is F and R_(f) ² is F, then at least one of R_(h) ¹, R_(h) ² andR_(h) ³ is C_(x)H_(2x+1).

In Formula I, A can be a single covalent bond or CH₂O. Preferably, inFormula I when A is CH₂O, then Y is a single covalent bond.

More particular cyclic carbonate electrolyte solvent materials are shownin Formula II and III, below, wherein the substituents are as definedabove.

In another aspect, the present invention provides acyclic carbonatecompositions having structures according to Formula IV:R—O—C(═O)—O—R′  IV.

One more particular embodiment wherein R is R_(h) ⁶, and R′ is C(R_(h)⁷)(R_(h) ⁸)_(n)CFR_(f) ¹(CF₂CF₂)_(n)CHFR_(f) ⁷ involves the compositionof Formula V:

In Formula V, R_(h) ⁶ is C_(x)H_(2x+1) and x is an integer from 1 to 4.R_(h) ⁷ and R_(h) ⁸ are independently hydrogen or an alkyl groupcomprising 1 to 4 carbon atoms and n is 0 or 1. R_(f) ⁷ can be F, alinear or branched C_(p)F_(2p+1) group, where p is an integer from oneto four or R_(f) ⁷ can be R_(f) ³O(R_(f) ⁴O)_(m)—. When R_(h) ⁷ andR_(h) ⁸ are both hydrogen, then n is 0 and R_(f) ⁷ is either a linear orbranched C_(n)F_(2n+1) group, where n is an integer from one to four orR_(f) ⁷ is R_(f) ³O(R_(f) ⁴O)_(m)—. When at least one of R_(h) ⁷ andR_(h) ⁸ is an alkyl group, then n may be 0 or 1. When n is 1, then atleast one of R_(h) ⁷ and R_(h) ⁸ is an alkyl group and R_(f) ¹ and R_(f)⁷ are both F. When at least one of R_(h) ⁷ and R_(h) ⁸ is an alkylgroup, and n is 0, then R_(f) ¹ is F, or C_(n)F_(2n+1) and R_(f) ⁷ is F,C_(p)F_(2p+1) or R_(f) ³O(R_(f) ⁴O)_(m)—, as described above. When bothR_(h) ⁷ and R_(h) ⁸ are alkyl groups, they may together form a ringhaving from 5 to 6 carbon atoms. A few exemplary electrolyte solventstructures are shown below in Formulas VI through VIII:

In another embodiment of ROC(O)OR′, R and R′ may each be independentlyrepresented by group C(R_(h) ⁷)(R_(h) ⁸)_(n)CFR_(f) ¹(CF₂CF₂)CHFR_(f) ⁷.The structures represented by this embodiment may be symmetrical when Rand R′ are the same, or the structures may be asymmetrical (when R andR′ are different). In each of the groups R and R′, each R_(h) ⁷ and eachR_(h) ⁸ is independently a hydrogen or an alkyl group comprising 1 to 4carbon atoms, n may be zero or one, and at least one of R_(h) ⁷ andR_(h) ⁸ in R or R′ is an alkyl group. When n is one then R_(f) ¹ andR_(f) ⁷ are both F. Where both R_(h) ⁷ and R_(h) ⁸ are alkyl groups,they may be linked to form a ring of from 5 to 6 carbon atoms.

In another aspect, provided is an electrolyte solvent compositionaccording to Formula IX:

wherein each substituent is as defined above.

A few exemplary electrolyte solvent structures are shown below inFormulas X and XI:

Lithium salts can be used together with the solvents disclosed herein.For example. Exemplary lithium salts are stable and soluble in thechosen charge-carrying media and perform well in the chosen lithium-ioncell, and include LiPF₆, LiBF₄, LiClO₄, lithium bis(oxalato)borate(“LiBOB”), LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, LiAsF₆, LiC(SO₂CF₃)₃, andcombinations thereof. Suitable cosolvents for mixing with the solventsof the present invention include those useful in lithium ion batteries,such as ethylene carbonate, propylene carbonate, dimethyl carbonate,diethyl carbonate, ethylmethyl carbonate, vinylene carbonate,vinylethylene carbonate, fluoroethylene carbonate or combinationthereof.

Rechargeable lithium-ion cells or batteries generally comprise apositive electrode and a negative electrode with a separatortherebetween, together with a charge-carrying electrolyte comprising acharge-carrying media or solvent or solvent mixture as provided in thisdisclosure along with a lithium salt. Suitable lithium salts include,for example, those described in the preceding paragraph.

Objects and advantages of this invention are further illustrated by thefollowing examples, but the particular materials and amounts thereofrecited in these examples, as well as other conditions and details,should not be construed to unduly limit this invention.

EXAMPLES

All numbers are weight percent (wt. %) unless otherwise noted.

Test Methods

A. Flammability

The flammability of each compound was tested by placing approximately 5mL of the compound in an aluminum weighing pan at room temperature andbringing a lighted wooden match to within about 1 mm of the samplesurface. If the sample ignited it was given a rating of Yes (a). If thesample did not ignite after about ten seconds, the sample was thencontacted with the lighted match. If the sample ignited, it was given arating of Yes (b), but none of the samples earned this rating. If it didnot ignite it was given a rating of No (b).

B. Solubility of LiPF₆

The proper proportions of LiPF₆ and the sample compound were combined atroom temperature to make a solution containing 1.0 mole of LiPF₆ in 1.0liter of the sample compound. If all of the LiPF₆ dissolved in thesample fluorinated compound, the LiPF₆ solubility is reported as “Yes”.If a major fraction of the LiPF₆ remained undissolved, the solubility isreported as “No”.

C. Accelerating Rate Calorimeter Exotherm Onset Temperature forLithiated Silicon and Lithiated Graphite with Electrolytes.

C.1. Preparation of Lithiated Silicon and Lithiated Graphite

Silicon (−325 mesh, 1.04 m²/g BET surface area, Sigma-Aldrich, St.Louis, Mo.) (80%), Super-S carbon (12%, MMM Carbon, Belgium),polyvinylidene difluoride (PVDF), (8%, a 9% by weight solution inN-methylpyrrolidinone (NMP), NRC, Ottawa, Canada)) and NMP, (twice themass of the silicon, Sigma-Aldrich) were milled into a slurry bymechanical milling in a modified Spex-8000 ball mill (including steelballs, 12.5 mm diameter, 1:10 mass ratio of silicon to steel balls) for30 min. using 500 shakes/min. The slurry was spread in a thick layer ona glass plate and was heated for about 12 h in an oven set at 105° C.The dried coating was removed from the glass plate, ground in a mortarand pestle and the powder was passed through a 300 μm (50 mesh) screen.The BET surface area of this silicon electrode powder was 3.82 m²/g. Thesilicon electrode powder (80±1 mg) and a circular piece of stainlesssteel mesh was pressed (96.5 MPa) into a silicon electrode pellet (14 mmdiameter, 0.32 mm thick).

Ethylene Carbonate (EC), Diethyl Carbonate (DEC) and LithiumHexafluorophosphate (LiPF₆) were obtained from EM Science (Gibbstown,N.J.). A 1 Molar (M) solution of LiPF₆ was prepared by dissolving theappropriate amount of LiPF₆ into the EC:DEC (1:2, v/v) solvent. Lithiumbis(oxolato)borate (LiBOB) (Chemetall GmbH, Frankfurt, Germany), wasdissolved in EC:DEC (1:2, v/v) to a saturated concentration of about0.8M.

Coin cells (2325) were prepared with a silicon electrode pellet, lithiummetal foil and 1M LiPF₆ EC:DEC (1:2 v/v) electrolyte and a separator.The cells were discharged using capacity control to form nominal Li₁Si,Li₂Si and Li₃Si (including irreversible capacity) using a currentdensity of about 6.5 mA/g. After the discharge was completed, the coincells were opened in an argon-filled glove box. The silicon electrodepellet was rinsed four times with dimethyl carbonate (DMC) and driedunder reduced pressure to remove the residual DMC solvent.

Mesocarbon microbeads (MCMB) (E-One Moli/Energy Canada Ltd., Vancouver,BC), that had been heat-treated to about 2650° C., were used to preparea graphite electrode powder by the same procedure used to prepare thesilicon electrode powder. The average particle size of the MCMB wasabout 20 μm determined by scanning electron microscopy (SEM), and theBET specific surface area was 0.81 m²/g. The BET surface area of thegraphite electrode powder, after drying and sieving, was 1.82 m²/g.Graphite electrode pellets (14 mm in diameter and 1 mm in thickness)were made using the same procedure as for the silicon electrode pellets,except that the graphite electrode pellets were about 300 mg in mass anddid not include the stainless steel mesh. Coin cells assembled with thegraphite electrode pellet, lithium metal foil, separator and 1M LiPF₆EC:DEC electrolyte were discharged to form nominal Li_(0.81)C₆.

C.2. ARC Sample Preparation

The samples for the ARC Exotherm Onset Temperature test were prepared asdescribed in Y. Wang, et al., Electrochemical and Solid State Letters,9(7), A340-A343, (2006). The sample holder was made from 304 stainlesssteel seamless tubing with a wall thickness of 0.015 mm (0.006 inches)(Microgroup, Medway, Mass.). The outer diameter of the tubing was 6.35mm (0.250 inches) and the length of pieces cut for the ARC sampleholders was 39.1 mm (1.540 inches). The temperature of the ARC was setto 100° C. to start the test. The sample was equilibrated for 15 min.,and the self-heating rate was measured over a period of 10 min. If theself-heating rate was less than 0.04° C./min., the sample temperaturewas increased by 10° C., at a heating rate of 5° C./min. The sample wasequilibrated at this new temperature for 15 min., and the self-heatingrate was again measured. The ARC Exotherm Onset Temperature was recordedwhen the self-heating rate was sustained above 0.04° C./min. The testwas stopped when the sample temperature reached 350° C. or theself-heating rate exceeded 20° C./min. The results of these ARC testswith electrolytes prepared with Examples 1-4 and with ComparativeExample 2 are displayed in Table 1.

D. ARC Exotherm Onset Temperature with delithiated LiCoO₂ anddelithiated LiMn₂O₄ with Electrolytes.

LiCoO₂ (particle diameter approximately 5 μm) and LiMn₂O₄ were obtainedfrom E-One Moli/Energy Canada Ltd. (Vancouver, BC). LiPF₆ (StellaChemifa Corp., Osaka, Japan) was dissolved to a concentration of 1M inEC:DEC (1:2 v/v) solvent (available from Ferro Corp., Cleveland, Ohio).

A sample of delithiated LiCoO₂ was prepared in a standard 2325 coincell. LiCoO₂ electrode powder was prepared by combining 7% each of SuperS Carbon Black and PVDF, (10% in NMP, NRC, Ottawa, Canada) with about300 mg of the LiCoO₂ powder. The NMP solvent was evaporated by heatingthe mixture at approximately 100° C. and the LiCoO₂ electrode powder wasforced through a 50 mesh screen. LiCoO₂ electrode pellets were preparedby pressing the powder in a die. The pellets were about 18 mm indiameter and about 1 mm thick.

Coin cells (2325) with a LiCoO₂ electrode pellet, a stainless steel meshelectrode, a 1 M LiPF₆ EC/DEC (1:2 v/v) electrolyte and threepolypropylene No. 2502 separators (Celanese Corp., Dallas, Tex.) wereassembled in an argon-filled glove box. The cells were charged with aconstant current of 1.0 mA until the cell voltage was 4.2 V vs. Li/Li⁺.After reaching 4.2 V, the cells were allowed to remain at open circuitconditions for 30 minutes. The open circuit voltage was less than 4.2 V.The cells were charged with a constant current of 0.5 mA to 4.2 V versusLi/Li⁺. After a total of four such charge-rest cycles, where the currentin each cycle was reduced by 50% from the current used in the previouscycle, the charged cells were dissembled in a argon filled glove box.The delithiated LiCoO₂ electrode pellet was removed from the cell andwas rinsed with dimethylcarbonate (DMC) four times. The pellet wasplaced in the glove box antechamber under reduced pressure for 2 h toremove the residual DMC. Finally, the sample was lightly ground for usein the ARC test.

A sample of delithiated LiMn₂O₄ was prepared by the same procedure asthat used to prepare the delithiated LiCoO₂, but substituting LiMn₂O₄powder.

The samples for the ARC Exotherm Onset Temperature test with delithiatedLiCoO₂ and delithiated LiMn₂O₄ were prepared in the same manner as thesamples for the ARC Exotherm Onset Temperature test with lithiatedsilicon, except that about 100 mg of either delithiated LiCoO₂ ordelithiated LiMn₂O₄ was used with an equal mass of test electrolyte, andthe temperature of the ARC was initially set to 110° C.

E. Charge Discharge Cycling of LiFePO₄/Li₄Ti₅O₁₂ and LiFePO₄/GraphiteCells Containing 0.5 M LiPF₆ Electrolytes.

Examples 1-4 were each tested neat in LiFePO₄/graphite andLiFePO₄/Li_(4/3)Ti_(5/3)O₄ coin cells. LiFePO₄ (from Phostech Lithium,Montreal, Canada). Li_(4/3)Ti_(5/3)O₄ was obtained from NEI Corp.(Piscataway, N.J.). The graphite used was mesocarbon microbeads (MCMB)heat treated to near 2650° C. Electrodes were made from the activematerials, 10% by weight Super S Carbon Black and 10% by weight PVDFbinder. LiFePO₄ and Li_(4/3)Ti_(5/3)O₄ electrodes were coated onaluminum foil and MCMB electrodes were coated on copper foil. A 20%capacity excess of the negative electrode was used in the cells, toensure that the negative electrode had a stable and known potentialversus Li/Li⁺ when the Li-ion cell reached the fully charged state(i.e., Li₀FePO₄).

The electrolyte formulations were 0.5 M LiPF₆ in each of Examples 1-4and Comparative Example 1. Coin cells (2325) employing Celgard No. 2502separators were used as test vehicles. Cells were charged using currentscorresponding to a normal recharge in 10, 20 or 40 h (C/10, C/20 orC/40) between the limits of 4.2 V and 2.5 V versus Li/Li⁺.

Initial experiments identified those solvents that could sustainacceptable charge-discharge cycling. There was some indication that someof the electrolytes may not have fully wetted the separator andelectrodes. Therefore experiments were repeated for the Example 1 wherethe electrodes and separators were “pressure wet”. Pressure wetting wascarried out by submerging the electrodes and separators in a vial ofelectrolyte, using a stainless steel weight to prevent floating. Thevial was placed within a vacuum/pressure vessel, and the vessel wasevacuated to approx-25 psi (172 kPa). After 30 seconds, the vessel wasslowly pressurized to 120 psi (827 kPa). After an additional 30 seconds,the pressure was slowly released and the electrodes and separators wereremoved. Coin cells were then made with no additional electrolyte added.

F. Cycling of LiFePO₄/Silicon Alloy cells with 10% of FluorinatedSolvent in EC:DEC Base Cosolvent.

Preparation of Silicon Alloy

Aluminum, silicon, iron, titanium, and tin were obtained as pureelements in high purity (99.8 weight percent or greater) from AlfaAesar, Ward Hill, Mass. or Aldrich, Milwaukee, Wis. A mixture of rareearth elements, also known as mischmetal (MM), was also obtained fromAlfa Aesar with 99.0 weight percent minimum rare earth content whichcontained approximately 50% cerium, 18% neodymium, 6% praseodymium, 22%lanthanum, and 4% other rare earth elements.

The silicon alloy composition was prepared by melting a mixture of 7.89g aluminum shot, 35.18 g silicon flakes, 9.34 g iron shot, 1.00 gtitanium granules, 17.35 g tin shot, and 29.26 g mischmetal (MM) in anargon-filled arc furnace (from Advanced Vacuum Systems, Ayer, Mass.)with a copper hearth to produce an ingot. The ingot was cut into stripsusing a diamond blade wet saw.

The ingots were then further processed by melt spinning. The meltspinning apparatus included a vacuum chamber having a cylindrical quartzglass crucible (16 mm internal diameter and 140 mm length) with a 0.35mm orifice that was positioned above a rotating cooling wheel. Therotating cooling wheel (10 mm thick and 203 mm diameter) was fabricatedfrom a copper alloy (Ni—Si—Cr—Cu C18000 alloy from Nonferrous Products,Inc., Franklin, Ind.). Before processing, the edge surface of thecooling wheel was polished with a rubbing compound (ImperialMicrofinishing from 3M, St. Paul, Minn.) and then wiped with mineral oilto leave a thin film.

After placing a 20 g ingot strip in the crucible, the system was firstevacuated to 80 milliTorr (mT) and then filled with helium gas to 200 T.The ingot was melted using radio frequency induction. As the temperaturereached 1350° C., 400 T helium pressure was applied to the surface ofthe molten alloy composition and the alloy composition was extrudedthrough a nozzle onto the spinning (5031 revolutions per min.) coolingwheel. Ribbon strips were formed that had a width of 1 mm and athickness of 10 μm. The ribbon strips were annealed at 200° C. for 2.5 hunder an argon atmosphere in a tube furnace, then cooled and powdered.

Each of Examples 1-4 was used as an additive to the electrolyte in coincells (2325) with an electrode of LiFePO₄ (from Phostech Lithium,Montreal, PQ, Canada), an electrode of the silicon alloy composition andNo. 2502 Celgard separators. The silicon alloy electrodes were 92%silicon alloy, 2.2% Ketjen black, 5.5% polyimide PI2555 (HDMicrosystems, Parlin, N.J.). The Ketjen Black and silicon alloy anodewere premilled together using a Fritsch Micromill Pulverisette 7(Goshen, N.Y.) planetary mill with four 13 micrometer diameter, tungstencarbide balls for thirty minutes at a setting of seven. LiFePO₄electrodes were made from the active material, 4% Super S Carbon Blackand 6% PVDF binder. LiFePO₄ electrodes were coated on aluminum foil. A10% capacity excess of the negative electrode was used. This was toensure that the negative electrode had a stable and known potentialversus Li/Li⁺ when the Li-ion cell reached the fully charged state(i.e., Li_(o)FePO₄).

Electrolytes used in the cells were 1M LiPF₆ in EC/DEC (1:2, v/v), with10% of Examples 1-4.

The coin cells were charged and discharged between 2.5V and 3.7V using aC/10 rate (based on the positive electrode) for the first lithiation ofthe silicon alloy. For subsequent discharge/charge cycles, a C/5 rate(based on the positive electrode) was used. A trickle current of 10 mA/g(based on the active electrode mass of silicon alloy) for lithiation ofthe silicon alloy was used at the end of each C/5 charge of the cell.The cells remained at open-circuit for 15 min. rest between each chargeand discharge.

Example 1 4-(1,1,2,3,3,3-hexafluoropropyl)-1,3-dioxolan-2-one

Example 1 was prepared by combining 2,2-dimethyl-1,3-dioxolane (200 g,1.96 moles, Fluka Chemie GmbH, Deisenhofen, Germany) and Luperox 575 (10g, 0.041 moles) (Arkema, Oakville, Ontario, Canada) were combined in a600 mL Parr reactor (Parr Instrument Co., Moline, Ill.). The reactor waswarmed to 75° C. and hexafluoropropene (300 g, 2.0 moles) (MDAManufacturing Inc., Decatur, Ala.) was added at a constant rate. Thereactor was allowed to stir at this temperature for 16 h. The crudereaction material was distilled using a ten plate Oldershaw column toafford 4-(1,1,2,3,3,3-hexafluoropropyl)-1,3-dioxolane (boiling point(b.p.) of 144° C.).

The above purified product (265.3 g, 1.05 moles) was combined withmethanol (674 g, 21.03 moles) and concentrated hydrochloric acid (26.03g, 0.71 moles) in a 3-L round bottom flask and the mixture was heated atreflux for 72 h. Excess methanol was removed by rotary evaporation. Theresulting fluorochemical diol (116 g, 0.55 moles) was combined withpyridine (181 g, 2.3 moles) and dichloromethane (235 mL) in a 1-L roundbottom flask. The temperature was maintained below 0° C. and phosgenesolution (20% in toluene, 60 g phosgene, 0.60 moles) (Sigma-Aldrich, St.Louis, Mo.) was added dropwise from a jacketed addition funnel that wasmaintained at or below 0° C. Following the complete addition of thephosgene solution, the reaction mixture was allowed to warm to roomtemperature. The reaction mixture was quenched with excess saturatedammonium chloride. The organic phase was collected. The aqueous phasewas extracted once with dichloromethane (200 mL). The organic phaseswere combined and washed with 1N hydrochloric acid, saturated, aqueoussodium hydrogen carbonate, saturated brine, dried with anhydrous sodiumsulfate, filtered and concentrated via rotary evaporation. Components ofthe reaction mixture were separated on a one-plate distillation column.

The purified product was analyzed by Gas Chromatography/MassSpectrometry (GC/MS). MCMB/LiFePO₄ coin cells (2325) made and cycled asper Test Method E with Example 1 as the only electrolyte solvent had acapacity fade rate of 2.65% capacity fade/cycle for a discharge rate ofC/40 and a capacity fade rate of 0.62% capacity fade/cycle for adischarge rate of C/10.

Example 2 4-((1,1,2,3,3,3-hexafluoropropoxy)methyl)-1,3-dioxolan-2-one

Example 2 was prepared by combining 4-(hydroxymethyl)-1,3-dioxolan-2-one(100 g, 0.84 moles) (Huntsman, Salt Lake City, Utah) with potassiumcarbonate (23 g, 0.166 moles) and acetonitrile (200 mL) in a 600 mL Parrreactor. The reactor was warmed to 45° C. and hexafluoropropene (139 g,0.92 moles) was added at a constant rate. The reactor was stirred untilthe decrease in pressure stopped. Volatile material was removed from thereaction mixture by rotary evaporation. The olefin of the desiredproduct was present and was saturated by reaction with anhydrous HF atroom temperature. Vacuum distillation was done to purify the product.The fraction that boiled at 145-150° C. at 3 T (0.4 kPa) was collected.The fraction was 97.5% pure. The product was analyzed by GC/MS and F-19NMR. MCMB/LiFePO₄ coin cells (2325) made and cycled as per Test Method Ewith Example 2 as the only electrolyte solvent had high impedance, andthe capacity fade rate could not be calculated.

Example 3 Ethyl 2,2,3,4,4,4-hexafluorobutyl carbonate

Example 3 was prepared by combining 2,2,3,4,4,4-hexafluorobutan-1-ol(184 g, 1.012 moles, Lancaster Synthesis Ltd., Ward Hill, Mass.),triethylamine (102 g, 1.008 moles) and methyl-t-butyl ether (350 mL) ina 1-L round bottom flask that was maintained at a temperature between 5°C. and 15° C. with a carbon dioxide/water bath. To the stirred mix,ethylchloroformate (100 g, 0.92 moles) was added from a jacketedaddition funnel that was maintained between 5° C. and 15° C. Theethylchloroformate was added over a period of 4 h. Once addition wascomplete, the reaction mixture was stirred for an additional 16 h andwas allowed to warm to room temperature. Then 100 mL of distilled waterwas added to the reaction mixture. The organic phase was collected. Thewater phase was extracted twice with 100 mL portions of methyl-t-butylether and all of the organic phases were combined. The organic phase waswashed with a 100 mL portion of distilled water and a 100 mL portion of1N HCl. The ether was removed by rotary evaporation. The remainingsample was purified by fractional distillation, using a concentric tubecolumn. The product was analyzed by GC/MS.

Example 4 Methyl-2,2,3,4,4,4-hexafluorobutyl carbonate

In a predried, two-necked, 500 mL round bottom flask, flushed withnitrogen, and equipped with a thermocouple probe, Claisen adapter,magnetic stir bar, water-cooled condenser and addition funnel,2,2,3,4,4,4-hexafluorobutan-1-ol (90.00 g, 0.4943 moles, LancasterSynthesis Ltd., Ward Hill, Mass.), triethylamine (62.7 g, 0.6196 moles)and methyl-t-butyl ether (200 mL) were combined. At an initialtemperature of 22° C., methylchloroformate (64.12 g, 0.6678 moles) wasadded dropwise, from the addition funnel, over a 1 h period. During theaddition, the temperature rose to 60° C. A white precipitate formedduring the reaction. After the complete addition of themethylchloroformate, the reaction mixture was stirred for about 18 h atambient temperature. The reaction mixture was combined, with stirring,with a premixed solution of 200 mL of 1.023 N HCl and 300 mL ofdeionized water. The resulting mixture separated into two phases. Theorganic phase was washed sequentially with 400 mL of water, 400 mL of 5%Na₂CO_(3(aq)), and two 400 mL portions of water. The organic phase wastreated for 3 days with activated 3 A molecular sieves. The product wascollected by fractional distillation, under nitrogen, at atmosphericpressure and a head temperature of 151.2-153.0° C. The product wasanalyzed by GC/MS and the purity was measured as 98.9% by GC-FID.

Each of the Examples 1-4 was rated “Yes” for solubility of LiPF₆.

TABLE 1 Results ARC Exotherm Onset Temperature Cell Charged ChargedCharged Charged Cycle Li_(X)Si LiCoO₂ Graphite Li₂MnO₄ Example FlammableAdditive (a) Anode (b) Cathode Anode Cathode 1 No (b) similar (+) 230°C. 175° C. 210° C. to 1:2 EC/DEC 2 No (b) similar (+) 230° C. 210° C.220° C. to 1:2 EC/DEC 3 No (b) Lower (=) 210° C. No Data 130° C. than1:2 EC/DEC 4 No (b) Lower (=) 240° C. No Data 150° C. than 1:2 EC/DECComparative Yes (a) 46% (−) 140° C.  90° C. No Data Example Capacity(1:2 retention EC:DEC) at 100th cycle EC No (b) (=) 140° C. 150° C.Notes: (a) Cell Cycle Life as 10% Additive to 1:2 EC:DEC (1M LiPF6),Silicon Alloy/LiFePO4 positive (b) (+) means that the Exotherm OnsetTemperature is higher than when EC is used, (=) means that the ExothermOnset Temperature is the same as when EC is used same as EC, (−) meansthat the Exotherm Onset Temperature is lower than when EC is used.

Example 5 Bis(3,3,4,5,5,5-hexafluoropentan-2-yl) carbonate Preparationof 3,3,4,5,5,5-hexafluoropentan-2-ol

A quantity of 3,3,4,5,5,5-hexafluoropentan-2-ol was prepared bycombining absolute ethanol 200 proof (100 g, 2.17 moles, AAPER Alcoholand Chemical Co.) with t-amyl peroxy-2-ethylhexanoate (5 g, 0.021 moles)(available as Luperox 575 from Arkema, Oakville, Ontario, Canada) in a600 mL Parr pressure reactor. The reactor was heated to 75° C. andhexafluoropropene (120 g, 0.8 moles) (MDA Manufacturing Inc., Decatur,Ala.) was added at a constant rate. The reactor was allowed to stir atthis temperature for 16 h. The crude reaction material was distilledusing a ten plate Oldershaw column to afford 150 g of3,3,4,5,5,5-hexafluoropentan-2-ol (b.p.=121° C.).

In a 1-L, 3-neck round bottom flask, 3,3,4,5,5,5-hexafluoropentan-2-ol(150 g, 0.76 moles) was combined with pyridine (120 g, 1.51 moles) and200 mL of methylene chloride. The mixture was stirred using an overheadstirrer and kept at a temperature of −15° C. using an ethyleneglycol/CO₂ bath. Triphosgene (40 g, 0.13 moles) (TCI America, Portland,Oreg.) was dissolved in 200 mL of methylene chloride and added to themix using a jacketed addition funnel maintained at −15° C. Following theaddition of the triphosgene, the cooling bath was removed and thereaction mixture was allowed to warm to room temperature over a periodof 24 h. Saturated aqueous ammonium chloride (250 mL) was added to thereaction mixture. The organic phase was collected. The aqueous phase wasextracted with one 200 mL portion of methylene chloride. The organicportions were combined and washed with a 100 mL portion of 1N HCl, 100mL of saturated sodium hydrogen carbonate and 100 mL portion ofdeionized water. A volatile fraction was removed by rotary evaporation.The remaining fraction was distilled and the fraction boiling at105-107° C. and 2.66 kPa was collected. The product was verified byGC/MS.

Example 6 4-(1,1,2,3,3,3-hexafluoropropyl)-1,3-dioxan-2-one

Example 6 was prepared by combining 1,3-dioxan-2-one (240 g, 2.35 moles)(Richman Chemical Inc., Lower Gwynedd, Pa.) with benzoyl peroxide (5 g,0.02 moles) in a 600 mL Parr reactor. The reactor was heated to 73° C.and hexafluoropropene (16 g, 0.1066 moles) was fed into the reactor overa 2 h period. The reactor was maintained at the set temperature for anadditional 24 h. The reactor contents were collected and analyzed byGC/MS.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand principles of this invention, and it should be understood that thisinvention is not to be unduly limited to the illustrative embodimentsset forth hereinabove.

The invention claimed is:
 1. A lithium ion battery comprising: a liquidelectrolyte; a positive electrode; and a negative electrode, whereinsaid liquid electrolyte comprises a solvent and a lithium salt whereinthe solvent comprises a compound according to the formulaR—O—C(═O)—O—R′, wherein R is C_(x)H_(2x+1) and x is an integer from 1 to4, and wherein R′ is —C(R_(h) ⁷)(R_(h) ⁸)CFR_(f) ¹(CF₂CF₂)_(n)CHFR_(f)⁷, wherein R_(f) ¹ is fluorine or C_(p)F_(2p+1) and p is an integer from1 to 4, R_(f) ⁷ is F, C_(p)F_(2p+1), or R_(f) ³O(R_(f) ⁴O)_(m)—, whereinR_(f) ³ is C_(y)F_(2y+1), and y is an integer from 1 to 8, and R_(f) ⁴is C_(q)F_(2q), wherein q is an integer from 1 to 4, and R_(h) ⁷ andR_(h) ⁸ are independently H or an alkyl group and wherein R_(h) ⁷ andR_(h) ⁸ are not both alkyl groups, the alkyl groups having 1 to 4 carbonatoms, m is 0 or 1, and n is 0 or 1, and such that when R_(h) ⁷ andR_(h) ⁸ are both hydrogen and n is 0, then R_(f) ⁷ is not fluorine. 2.The lithium ion battery of claim 1 wherein R_(h) ⁷ and R_(h) ⁸ are bothhydrogen and n is zero.
 3. The lithium ion battery of claim 1 whereinone of R_(h) ⁷ and R_(h) ⁸ is an alkyl group, n is 1, and both R_(f) ¹and R_(f) ⁷ are fluorine.
 4. The lithium ion battery of claim 1 whereinone of R_(h) ⁷ and R_(h) ⁸ is an alkyl group, n is zero, and R_(f) ¹ isF or C_(n)F_(2n+1), and R_(f) ⁷ and R_(h) ⁸ are F, C_(n)F_(2n+1), orR_(f) ³O(R_(f) ⁴O)_(m)—.